38 GEOLOGY OF THE EUREKA DISTRICT. 



of magnesia varies from 1-06 to 44-35 per ceut; thi-ee of them yielded less 

 tlian 2 per cent. In nine out of the sixteen the amount of the silica in the 

 limestone was less than 2 per cent. 



Many of the beds, more especially the darker limestones, give evidence 

 of the presence of organic matter, even where no signs of fossils are seen. 

 Proof of this is found in the presence of phosphoric acid in the rock. Two 

 specimens yielded OlS per cent, evidently derived from the fossil remains 

 now almost wholly obliterated. 



Sandstone layers are rarely seen in this group. Interstratified in the 

 limestone are irregular beds of shale, lenticular or wedge-shaped bodies 

 varying greatly in width. Indeed, throughout the entire thickness of this 

 group they are a characteristic feature of the beds, which pass by insensible 

 gradations from pure limestone to hard argillaceous shales. Occasionally 

 they may be traced interstratified in parallel bands for long distances, and 

 again the shale will develop considerable thickness, then rapidly thin out 

 in all directions. For the most part they can be followed for no great dis- 

 tance. Two of these shale beds are quite distinctly marked on the top of 

 the ridge to the northward of Prospect Peak, but all traces are lost on the 

 surface to the south of that point. One of these shale beds on the east slope, 

 however, attains so great a thickness that it has been designated Moun- 

 tain shale, to distinguish it from the Secret Canyon horizon. Unlike the 

 larger body of overlying shale they are of slight geological significance, the 

 limestone both above and below presenting nearly identical physical fea- 

 tures, and so far as known canying the same organic forms. The Mountain 

 shale comes to the surface on the ridge near the Industry mine and on the 

 steep slope of the ridge above the Eureka Tunnel, across its widest devel- 

 opment reaching over 300 feet in thickness. It differs from the Secret 

 Canyon shale in can-ying alternate layers of argillaceous and calcareous 

 shales, the latter frequently passing into stratified shaly limestone. This 

 body of intercalated shale presents some features of ecotiomic interest bear- 

 ing upon the ore deposits, and may possibly be the same bed found in all 

 the deep mines on Ruby Hill. The thickness of the Prospect Mountain 

 limestone across its broadest expansion may be taken at 3,050 feet. On 

 Ruby Hill, owing to faulting, it never attains its full development. 



